A positive photoresist is applied on a substrate such as a semiconductor wafer, glass, a ceramic, or a metal by spin coating or roller coating at a thickness of from 0.5 to 2 .mu.m. The coating is dried by heating, and then irradiated with, e.g., ultraviolet ray through an exposure mask to cure the coating in the form of, e.g., a circuit pattern. If desired, the exposed coating is baked. Thereafter, the coating is developed to form a positive image.
By etching the substrate using this positive image as a mask, the surface of the substrate can be pattern-wise processed. Typical applications of the positive photoresist include manufacture of semiconductors such as IC and the like, manufacture of circuit boards for liquid crystals and thermal heads, and other photofabrication processes.
However, the degree of integration in integrated circuits is becoming higher increasingly, and the production of substrates for semiconductor circuits, e.g., VLSI's (very large scale integrated circuit), has come to necessitate a processing for forming an ultrafine pattern having a line width of 0.5 .mu.m or smaller. The photoresists for use in this application are required to attain high resolving power especially stably and have a wide development latitude so as to ensure a constant pattern line width always. The photoresists are also required not to leave a residue on the developed resist pattern so as to prevent the circuit from having processing defects.
On the other hand, it has been found that in the formation of an ultrafine pattern, in particular, one having a line width of 0.5 .mu.m or smaller, there is a phenomenon in which even though a certain degree of resolving power is obtained at a certain coating film thickness, this resolving power deteriorates upon a slight change in coating film thickness (hereinafter, this phenomenon is referred to as "film thickness dependence"). It has surprisingly been also found that when the film thickness changes by only a few hundredths of a micrometer, the resolving power changes considerably, and that all the representative positive working photoresists currently on the market more or less show this tendency. Specifically, when the thickness of an unexposed resist film varies from the intended film thickness in the range of .lambda./4n (.lambda. is exposure wavelength and n is the refractive index of the resist film at that wavelength), the resulting resolving power fluctuates accordingly.
The presence of this problem of film thickness dependence was pointed out in, e.g., SPIE Proceedings, Vol. 1925, p. 626 (1993), where there is a description to the effect that the film thickness dependence is caused by multiple light reflection in the resist film.
This film thickness dependence has been found to be enhanced in most cases particularly when resist contrast is heightened so as to obtain a high resolving power and a pattern having a rectangular section. In the actual processing of a semiconductor substrate, a pattern is formed from a resist film whose thickness slightly varies from part to part due to the surface roughness of the substrate and unevenness of coating thickness. Therefore, this film thickness dependence has been an obstacle to the pattern formation in which a positive working photoresist is used to conduct fine processing at a resolution close to its resolution limit.
Accordingly, the photoresists are required to exhibit high performances with respect to the above-mentioned various properties.
To improve the resolution, use of polyhydroxy compounds having various particular structures has been proposed so far. For example, a number of 1,2-naphthoquinonediazide compounds are proposed. These compounds are disclosed in, e.g., JP-A-57-63526, JP-A-60-163043, JP-A-62-10645, JP-A-62-10646, JP-A-62-150245, JP-A-63-220139, JP-A-64-76047, JP-A-1-189644, JP-A-2-285351, JP-A-2-296248, JP-A-2-296249, JP-A-3-48249, JP-A-3-48250, JP-A-3-158856, JP-A-3-228057, JP-A-4-502519 (Tokkohyo), U.S. Pat. Nos. 4,957,846, 4,992,356, 5,151,340, and 5,178,986, and European Patent 530 148. (The terms "JP-A" as used herein means an "unexamined published Japanese patent application".) However, these photosensitive material are insufficient from the standpoint of reducing the film thickness dependence.
Also, these compounds only brings about insufficient improvement in the above-mentioned various performances. Particularly, there is still a room to improve the sensitivity, resolution and the above-mentioned dependence on film thickness.